Measuring based on a capacitive acceleration sensor has proved to have a simple principle and to provide a reliable method for measuring acceleration. The capacitive measuring is based on a change in a gap between two surfaces of a pair of electrodes in the sensor. The capacitance between the surfaces, or the capacity for storing electric charge, depends on the area of the surfaces and the distance between the surfaces. Capacitive measuring can be used already at rather low measuring ranges of acceleration.
The prior art is described below with exemplifying reference to the enclosed FIG. 1, which shows a side view of the structure of a pair of electrodes of an acceleration sensor according to prior art.
FIG. 1 shows a side view of the structure of a pair of electrodes of an acceleration sensor according to prior art. The pair of electrodes of the acceleration sensor according to prior art comprises a movable electrode 1, which moves according to the acceleration, and a stationary electrode 2.
The movable electrode 1 is that part 1 of the acceleration sensor, which is responsive to acceleration, is supported by springs, and due to acceleration moves in relation to the stationary electrode 2. The movable electrode 1 and the stationary electrode 2 constitute a pair of electrodes converting acceleration into a quantity that can be measured electrically, i.e. capacitance. Generally, on the opposite side of the movable electrode 1, the acceleration sensor of prior art also comprises a second pair of electrodes, not shown in the figure for clarity reasons.
In measuring acceleration, the capacitance between the movable electrode 1 and the stationary electrode 2 of the pair of electrodes of the sensor, i.e. the capacity for storing electric charge, depends on the area of the surfaces and the distance between the surfaces. As acceleration through motion is applied to the object to be measured, the stationary electrode 2 follows the movement of the object to be measured. The movable electrode 1 tends to remain in its position due to forces of inertia, and thus moves in relation to the stationary electrode 2. The distance between the movable electrode 1 and the stationary electrode 2, and thus also the measurable capacitance, changes.
The pair of electrodes of a prior art acceleration sensor also comprises an isolator protrusion 3. The pair of electrodes may also have multiple isolator protrusions 3. An isolator protrusion limits the movement of the movable electrode 1, which is supported by springs, and prevents contact between the electrodes 1, 2, in case of overload, and thus also prevents shorting of the meter circuit. The use of an isolator protrusion is described in, for example, the publication U.S. Pat. No. 5,367,429.
Generally dielectric protrusions are used as isolator protrusions 3. In sensitive acceleration sensors the restoring spring forces of the structure are weak, such that surface forces may cause temporary or permanent sticking of the movable electrode 1 to the stationary electrode 2, thus preventing the functioning of the sensor.
In particular, efforts for preventing sticking in surface micro-mechanical acceleration sensors have been made by way of various roughening or coating treatments. For example, surface force reducing films of lipid films or fluoropolymers have been applied to the structures.
In the sensor structures relating to the present invention, there may further occur mechanical wear of the isolator protrusions during the production process. Thus scuffed oxide protrusions get deformed and they can even mechanically lock on to an equally worn silicon surface.